It is well known that temperature affects the performance of electrical circuitry and it is important to provide circuitry which provides an output which is not dependent on temperature fluctuations, i.e. a voltage reference. It will be appreciated that a voltage reference can be converted to current reference and for the sake of the following explanation the present teaching will be described with reference to the provision of a voltage reference at the output of the circuit but it will be understood that the present teaching should be construed as limited to such a voltage reference.
In the context of providing voltage references, it is known to use a band-gap type voltage reference which is based on a summation of two voltage components having opposite and balanced Temperature Coefficients (TCs). Usually, the first voltage component is related to a base-emitter voltage of a bipolar transistor which inherently has a form which is Complementary To Absolute Temperature, denoted as a CTAT voltage. The second voltage component is obtained from the base-emitter voltage difference, ΔVBE, of two bipolar transistors operating at different collector current densities. This voltage is Proportional To Absolute Temperature and it is denoted a PTAT voltage. Very often the base-emitter voltage difference is reflected over a resistor generating a corresponding PTAT current. With a second resistor of the same type (same TC) the base-emitter voltage difference is gained to the desired level to balance the CTAT voltage component.
A real voltage reference is affected by many errors such as temperature drift or temperature coefficient (TC). Such a variation in response with respect to operating temperature may be considered a first order variation but it is also possible for resultant errors to have a contribution from higher order error components. Such higher order errors can be very well approximated by a parabolic or second order form versus absolute temperature. To compensate for these errors there is always a need for a trimming circuit and a method to guarantee the target specifications independent of how the circuit is designed or its architecture.
In summary, there is a continuous need for circuits that can provide an accurate reference circuit.